How do I hide the red lines of the path following in the game engine?
In the top of the Blender window there should be a menu bar (called the Info Bar) with the drop-down menus for File, Game, Window, etc.

Click on Game, then make sure the option Show Physics Virtualization is unchecked. (If you are using the UPBGE fork, go to Game, then click the Show Debug option to open another submenu where the option is there.)

This may be the culprit. It can be easy to turn on by accident.